cylinder head inlet port to port centres

[WestOzFJ]

Am hoping someone knows or will measure the 1 to 2 and/or 3 to 4 cylinder head inlet port centres, (not concerned with the 2 to 3 split) - I'm on leave from work and don't have access to any of my measuring tools....

I'm keen on some throttle body paired blocks I've come up with while researching that are made on 80mm centres which is real close to what I can measure at the inlets externally on the head of a still assembled engine.

Can anyone confirm or refute the 80mm measurement with more accuracy than I have access to at the moment?

[andyb]

I can attempt to measure when I'm a bit more awake.*

Do you want the actual spacing at the head itself, or of the carb centerlines?  Guessing the latter if you're going to use the original intake rubbers, the former if you're going to go way out on a limb?

*I'm from the "measure twice, cut once, cut again, measure again, test fit, swear some, measure again, cut some more, swear more" school of working, so I can't promise perfection, but I'll give it a go.

[WestOzFJ]

Thanks in advance for offering to give it a shot at measuring....

As far as I can see the 36Y inlet rubbers for cylinders 1 and 2 are the same part as are 3 and 4 the same as each other. So if that's true then the measurement at the inlet port should be nigh on identical to the carb split. At the carb spacer bar/rack (that ties all 4 together) I measure about 78, yet at the inlet port head face I get 80.

At the inlet port of the head is probably most relevant it tells me what the base design is which ultimately after all the standard intake tract stuff could be shelved - that dimension of the head ports is probably where I'll be working outwards from.

So what is going to go between the head and throttle bodies is yet undecided, but 36Y inlet rubbers are possible, as are injected XJR alloy intake blocks with the injector boss already machined in (but not using Yamaha injectors sooo...), but as is usually the case with me I'll probably end up making something from scratch to get EXACTLY what I want, so next step is making contact with head gurus this week to find out if to persist with the dog leg inlet tract or take the opportunity to make straighter shot adaptors with the right maths applied to the shape to tidy up the airflow....

[RACER111V]

Maybe this will help.

http://www.carbparts.com/mikuni/mikuni_rs_standard_tuning_specif.htm

[WestOzFJ]

Yep, that link does help, thanks. I'll take that as the definitive answer.

Geeez now why couldn't I find that....

[fj1289]

Just took a set of calipers to two heads - one with the rubber carb boots on and one without.  To my surprise the offset was the same on both - I thought the outboard carb rubbers (#1 and 4) had a bigger offset - but it appears all the carb rubbers have the same amount of offset. 

The calipers and my eyeballs say 80mm is closer than 77mm from the Mikuni site.  There's enough flex in the boots to deal with both dimensions easily.

It will take some convincing for me to believe spigots coming staight off the head will give better performance - the reason is the intake ports in the head are already angled.  The factory boots run at the same angle as the ports in the head, so there is no additional turn made.  I think the performance improvement will come from boots/spigots that do not taper as much after the carb/throttle body and into the head.  The end of the carb rubber/opening of the head necks down to ~ 32.5mm. 

With FI, the velocity through the throttle body shouldn't be nearly as important as on a carb since you don't need a vacuum signal to meter the fuel.  Velocity through the ports is every bit as critical though.

So, in the end, I'd say go with what is the easiest, most direct route to get throttle bodies and fuel system set up.  You'll have enough to figure out with the required sensors and ECM/controller.  Getting that wired (pun intended!) will have a much bigger effect on driveablity and outright performance than will theoretical intake lengths, etc.

Chris

[WestOzFJ]

Cheers for that Chris, I get the same measurement and found it difficult to believe it's so far different to "that" mikuni number...

It's got me curious now as to what the engine designer had on paper, I want to make 4 cylinders that run the same so will have to pull covers off and makes some valve centre measurements - I think 1 or 2 mm does matter....

Everything you said makes logical sense to me including the matter of port curvature and my version 1 goal is to get a good solid, streetable, tractable and reliable package that moves the performance potential forward, although the word "performance" can be taken to mean many different things to different projects.

So I am hearing you - there'll be many more important details to get right for the road-going version without the added complexity of redesigning the inlet tract just for sake of it. But it's hard to not think about such things - I've had a lot of success with the "think outside the box" approach in the past.....

Thanks for the chat Chris,

Victor...

[andyb]

You shouldn't get into too much trouble.  Yes, the different length of the intake runners makes a tuning difference, but I have to think you're going to have a hard time screwing it up too badly.  Ideally you would put the stock intake rubber on the head and measure the volume of the entire port, that'll help give you a guideline on mounting the TB's. 

Are you looking to go significantly larger or smaller on the TB's?  You can always use a bit of heat and dilate the intake boots, probably cheat 2-3mm that way.  Further, depending on the TB's you use, it may be possible to trim their OD on the hot side a bit.  I have to think that if you're going to something huge, you're going to lose velocity to a harmful degree, and with it a fair bit of effectiveness.

If you're not building a wild motor, the rule seems to be that you are better off undersized than oversized on the intake tract... velocity makes more difference than absolute flow.

When measuring valve center distance to head entry, it's worse yet... you're going to have to look at swirl and the angle that the charge hits the valves.  Can get some shocking results when you manipulate the mixture to move around a bunch! 

[WestOzFJ]

Had a closer look at the head today, reveals port curvature is slightly different between 1 and 2, likewise 3 and 4. But that's okay, I can work with that and I'm not going to go manipulating the head flow to begin with as tempting as it is to try it.

I'm going to back to back test FI to typical kinda standard carbs on a kinda standard engine - that's the baseline, had to draw a line to start and make progress and that's it, where it goes to from there....

The ITB block pairs on 80mm centers I've been continually re-drawn to are 40mm butterflies. They have a very neat, tuneable ram tube arrangement, will take a TPS that's compatible with intended ECU, modular build as you go throttle linkages hookup the left to right pairs in the centre on whatever spacing is correct - that kind of constructability is very appealing. MOST of all I think the price of the bits to build is quite reasonable considering then I'd get exactly what I want rather than having to chop something up that doesn't fit right in the first place and there's just something screwed up about making a manifold to adapt to incorrect throttle body centres rather than building a body true to the engine dimensions....

But I'd still like to save some bucks if I can so...

If I could get some XJR TB's at a reasonable price they're the right centres... But they're as rare as rocking horse poo here in Oz. Going down the 36Y path limits to about 40mm which is probably about right for a road going version and the XJR alloy version of 36Y puts the injector closer to the port than I really want it but I'd work with that. I've seen them breaking injected XJR TB's in UK but with shipping costs.... PLUS, $AUD is much more favourable with USD than GBP. Maybe that's just as well, I'm not really that impressed with current XJR tuning anyhow. Over on XJRowners asking how to get 'em as powerful as FJ's, still. How the heck did that happen, STILL after 25 years?

Been down to see my local friendly breaker today, he's (only) got three ITB's - all complete with TPS, injectors, fuel rail, etc: Honda CBR 954 at 43mm, CBR600 at 38mm and GSXR 750 at 40mm so I've been researching these 3 this afternoon.

I asked him, "hows about a sniff of an R1 throttle body set" (just so as I could look see what all the hoohaa is with these - I mean even guys with busas are puttin' 'em on), you think I'd asked him for a sniff of his crotch, it just wasn't gonna happen...

Anyhow, back to research... Cheers.